The operation of a photovoltaic cell is mainly based on the absorption of photons generating the transiting of electrons between the valence band and the conduction band of a material having semiconductor properties, forming the cell. Such an electron transfer is made possible due to the doping of the material forming the photovoltaic cell, to create areas having an excess of electrons (n doping) as well as areas lacking electrons (p doping).
Generally, a photovoltaic cell comprises a p-doped silicon substrate covered with an n-doped silicon layer. Such a stack forms a pn junction necessary for the collection of photocarriers generated by the exposure of the photovoltaic cell to sunlight. The n-doped silicon layer is further covered with an antireflection layer ensuring a good photon absorption. The latter comprises electric contacts enabling to collect the generated current.
However, certain constraints should be respected, the n doping areas simultaneously having to:                ensure a good ohmic contact with the electric contacts, and thus have a high doping level;        ease the passivation of the material by means of the antireflection layer, and thus have a low doping rate to limit Auger recombinations, which are associated with a high doping rate.        
Accordingly, photovoltaic cells called selective-emitter cells have been developed. Such cells have areas of strong n doping as well as areas of low n doping in an n-type or p-type substrate.
Selective-emitter photovoltaic cells of prior art (FIG. 1F) thus comprise an area of low n doping, also called n+ area or region or n+ emitter. Further, in this type of photo-voltaic cell, the emitter also comprises areas of strong n doping (n++ regions or n++ emitters) precisely defined to form electric contacts. The n+ emitter thus eases the passivation by the antireflection layer and decreases Auger recombinations while the n++ emitter is connected to the electric contacts to provide a good ohmic contact.
Typically, methods for manufacturing such a photovoltaic cell with a selective emitter according to prior art comprise the steps of:                forming an n+ emitter by gaseous diffusion of a dopant (POCl3) in a p-doped silicon substrate (J. C.C. Tsai, “Shallow Phosphorus Diffusion Profiles in Silicon”, Proc. of the IEEE 57 (9), 1969, pp. 1499-1506). This step implies maintaining the substrate at a temperature close to from 850 to 950° C. for several tens of minutes (FIGS. 1A and 1B);        forming an n++ emitter by laser doping (A. Ogane et al, “Laser-Doping Technique Using Ultraviolet Laser for Shallow Doping in Crystalline Silicon Solar Cell Fabrication”, Jpn. J. Appl. Phys. 48 (2009) 071201), or by a second gaseous diffusion of a dopant (POCl3) at a temperature higher than that involved in the case of the n+ emitter (FIG. 1C). Certain specific areas of the n+ emitter will be overdoped;        deposition of an antireflection layer, typically silicon nitride, by PECVD (“Plasma Enhanced Chemical Vapor Deposition”) (FIG. 1D);        forming the electric contacts by:                    deposition of a metallization gate (n contact) on the upper surface of the substrate. It typically is a silk screening paste containing silver. The patterns of this metallization gate are precisely aligned on the n++ emitters to avoid short-circuiting the n+ emitter on annealing of the contacts (FIG. 1E). Indeed, if the metallization shifts above the n area, since the latter is thin, on anneal, the metal may cross it and put the n+ area in contact with the substrate.            deposition of a paste containing aluminum (p contact) over the entire lower surface of the substrate. It enables, on the one hand, to ensure the contact with the p-doped portion of the photovoltaic cell and, on the other hand, to improve the electric properties thereof by BSF (“Back Surface Field”), that is, by field-effect passivation (FIG. 1E). It is a heavily p-doped layer, enabling to repel electrons away from the surface and to decrease the electron-hole recombination speed, and this, to improve the cell efficiency:            forming of the electric contacts by simultaneous anneal of the pastes (silver and aluminum) in a continuous furnace, for example, at a 885° C. temperature and with a 6,500 mm/min belt speed (B. Sopori et al, “Fundamental mechanisms in the fire-through contact metallization of Si solar cells: a review”, 17th Workshop on Crystalline Silicon Solar Cells & Modules: Materials and Process, Vail, Colo., USA, Aug. 5-8 2007). The anneal step is highly critical, since it must be ensured that in a single step, a good ohmic contact is achieved at the upper and lower surfaces of the substrate, and that the passivation of the lower surface is performed by BSF effect (FIG. 1F).                        
In prior art methods, and as for example described in document FR 2943180 and WO 00/01019, the step of forming the n+ emitter and that of annealing the n contacts and the p contacts are incompatible and thus cannot be carried out simultaneously. Indeed, the step of forming the n+ emitter by gaseous diffusion of a dopant (POCl3) is relatively long (several tens of minutes), and would result in short-circuiting the areas under the electric contacts if it was performed after deposition of said contacts. On the other hand, the pastes used in silk screening, which is the method used to form the electric contacts, are incompatible with furnaces used for the diffusion of dopants, since they involve large quantities of metals which would irremediably pollute the diffusion furnaces.
Prior art methods thus comprise incompatible steps, each requiring a very specific energy input. On the contrary, the present invention enables to decrease such energy constraints by combining certain steps of the manufacturing of a selective-emitter photovoltaic cell.